BUG: Fixed piecewise function for 0d input

When `x` has more than one element the condlist `[True, False]`
is being made equivalent to `[[True, False]]`, which is correct.
However, when `x` is zero dimensional the expected condlist is
`[[True], [False]]`: this commit addresses the issue. Besides,
the documentation stated that there could be undefined values
but actually these are 0 by default: using `nan` would be desirable,
but for the moment the docs were corrected. Closes #331.
BLD: compute sha256 sums of build artifacts

Using a secure hash allows verifying ther binaries from a gpg signed
README.txt.
MD5 just kept for legacy users who may not be able to obtain a sha256
utility.
Merge pull request #4792 from Juanlu001/piecewise-fix

Piecewise fix for 0d
Merge pull request #4793 from juliantaylor/sha256-sums

BLD: compute sha256 sums of build artifacts